1. Field of the Invention
The present invention generally relates to illumination of image displays, and more particularly to illumination of image displays using holographic techniques.
2. Description of the Relevant Art
Proper operation of image displays such as flat panel displays is in part dependent upon the optical character and intensity of light illuminating the display device. Under conditions of sufficient ambient light, operation of flat panel displays may be adequate. However, reduced ambient light may diminish suitable contrast to viewers of the display device. Such reduced contrast is addressed in the prior art by supplying an internal, supplemental light means to enhance illumination and make the display more viewable. However, the incorporation of -supplemental light sources adds bulk to the display and the system thereof and increases power requirements. Many image display systems are power sensitive. Conventional systems employing, for example, flat panel displays (e.g., laptop computers and cellular telephone displays) are sensitive to power budget constraints. In these systems, supplemental light sources for back lighting and edge lighting are often times the greatest source of power drain. The problem is to capture ambient light from as large an area as possible, (e.g., from a full hemispherical region), and channel the captured light into useful directions for illuminating an image display without the need for a supplemental source of illumination.
The present invention relates to an apparatus and method for illuminating an image display using ambient light. The present invention employs holographic techniques for collecting ambient light over a hemispherical region and redirecting the collective light into useful viewing directions.
In one embodiment, the apparatus of the present invention comprises at least a first pair of holographic optical elements each having a first surface aligned on a common axis. The first pair of holographic optical elements are themselves positioned such that the first surfaces of each face each other. The first holographic optical element of the first pair is configured to diffract a select portion of received ambient light. More particularly, the first holographic optical element is configured to diffract ambient light within a first bandwidth. The second optical element of the first pair is likewise configured to diffract a select portion of light received thereon. More particularly, the second holographic optical element is configured to diffract first bandwidth light which is received on the first surface thereof. This light after being diffracted by the second holographic optical element emerges from the first surface of the second optical element. Both the first and second holographic optical elements are also configured to transmit light, other than the first bandwidth light, without substantial alteration. Lastly, a quarter wave plate is positioned between and aligned with the first surfaces of the first and second optical elements.
In another embodiment, the second holographic optical element is switchable between active and inactive states. The second holographic optical element transmits first bandwidth light substantially unaltered when operating in its inactive state. In contrast, the second holographic optical element diffracts first bandwidth light received on the first surface when operating in the active state.
In yet another embodiment, the first holographic optical element is switchable between active and inactive states. In the inactive state, the first holographic optical element is configured to transmit first bandwidth light substantially unaltered. In the active state, the first holographic optical element diffracts first bandwidth light.
In still another embodiment, the first or second holographic optical element is formed from polymer dispersed liquid crystal material. In this embodiment, the polymer dispersed liquid crystal material undergoes phase separation during the hologram recording process, creating regions densely populated by liquid crystal micro-droplets, interspersed by regions of clear photopolymer.
In yet another embodiment, the second holographic optical element comprises a layer of material that records a hologram and an array of electrically conductive elements, wherein the array of electrically conductive elements is positioned adjacent the layer of material that records the hologram.